production

We will produce Melt-blown non-woven fabric also known as filter material. 

The material is meant to be able to remove all kinds of pollutants from the air people breathe - such as viruses and bacteria, but also abrasives or other dusts. In order to filter tiny aerosol droplets such as micro-organisms, the fleece is required to have an extremely fine structure.

The Polypropylene (PP) resins are first melted until it has approximately the consistency of liquid honey, which flows through tiny nozzles and forms a wafer-thin thread underneath.

To achieve this final degree of slenderness, the melted thread is blown, in what is thus often called the melt-blown process –This process makes the melt spun fibre extremely thin with diameters in the sub-micron range without splitting.

Nanometer-thin threads, but extremely durable

The polypropylene has a melting point of 160 degrees Celsius. The air is about 250 degrees. The hot air and the hot melt meet under extreme acceleration. The air hits the melt threads at about 300 meters (980 feet) per second. The high speed makes the filaments extremely thin. 

The chaotic movements of the filaments help to form a tangled net, and due to polymer still being hot, it sticks together on the screen belt. This net has a physical pore size of about 10 micrometers. 

Viruses are much, much smaller still

Although it is already fine, the non-woven net would still not filter viruses out of the air. The openings in the filter material are about one hundred times as large as the virus, with its 0.12 micrometers. 

So, engineers use physical tricks, such as exploiting the tendency of small particles to attach themselves to surfaces. The particle attaches to the surface as it travels through. It hits a filament and gets caught on it because of frictional or intermolecular forces. Forces between molecules are important when we think in terms of viruses.

Electrostatic attraction

An additional force is introduced that separates the viruses and draws them in. These are electrostatic forces,  which are used extensively. 

Afterwards, the fleece is like an electrostatic charged mop, like dusting mops.  The filter material works the same way as they do.

This would apply to aerosol size droplets that somebody coughs out. As long as the viruses are already attached to those droplets or swimming in them, the filter catches them, even if they are tiny.